Batteries having zinc electrodes are known. See U.S. Pat. Nos. 4,359,510; 4,438,185; and 4,544,616; Tuck, C. D. S., Editor, Modern Battery Technology, Ellis Horwood Limited, Chichester, England (1991); and Linden, D., Editor, Handbook of Batteries, Second Edition, McGraw-Hill, Inc., New York, N.Y. (1995), each of which is incorporated herein by reference. One such battery is the nickel-zinc (NiZn) battery (or cell).
The nickel-zinc (zinc/nickel oxide) battery system includes a zinc electrode, a nickel electrode, an electrolyte, and a separator. The zinc/nickel oxide battery system uses zinc as the negative active material and nickel oxide as the positive, and the electrolyte is an alkaline potassium hydroxide solution.
While NiZn cells are well known, they have never reached a significant commercial success; apparently, because of the limited effective cycle life of such batteries. Besenhard, J., Editor, Handbook of Battery Materials, Wiley-VCH, New York, N.Y. (1999). The reason for this limited cycle life is the high solubility of the zinc hydroxide in alkaline electrolyte; the zincate ions formed are deposited again during the subsequent charging in the form of dendrites, i.e. fernlike crystals. See Besenhard, Ibid. These dendrites grow in the direction of counter electrode and finally cause electrical shorts. See Besenhard, Ibid.
A short term remedy can be achieved by a decrease in zinc solubility in the electrolyte or by suppression of dendrite formation; cadmium-, lead-, or bismuth oxide, as well as calcium hydroxide or aluminum hydroxide have been added to the zinc electrode or the electrolyte to suppress dendrite formation. See Besenhard, Ibid. However, this remedy does not have long-lasting effectiveness. See Besenhard, Ibid.
It has also been suggested that microporous films could be used to overcome the problems associated with the use of zinc electrodes. See Tuck, Ibid., Linden, Ibid., and U.S. Pat. Nos. 4,359,510; 4,438,185; and 4,544,616.
One commercially available microporous membrane for use in batteries having zinc electrodes is Celgard® 3406 microporous membrane. See: U.S. Pat. Nos. 4,359,510 and 4,438,185. Celgard® 3406 is a microporous membrane having a polymer coating on one surface. The microporous membrane is commercially available as Celgard® 2400 microporous membrane, a polypropylene microporous membrane having an average pore size of about 0.045 microns. The polymer coating consists of cellulose acetate and a surfactant commercially available as VICTAWET® 12 wetting agent. VICTAWET® 12, an oxirane polymer with 2-ethylhexyl dihydrogen phosphate, is commercially available from Akzo Chemicals, Inc., Chicago, Ill. While Celgard 3406 performs adequately, it has a limited shelf life (about 9 months from the coating date) and it wets only once (i.e., the surfactant readily washes off).
Another commercially available microporous membrane for use in batteries having zinc electrodes is Celgard® 3407 coated microporous membrane. See: U.S. Pat. No. 6,479,190. Celgard® 3407, which is commercially available from Celgard, Inc. of Charlotte, N.C., comprises a microporous membrane having a coating on at least one surface of the membrane. Celgard® 3407 microporous membrane, having an average pore size of about 0.045 microns, is typically a hydrophobic, polyolefinic polymer. The coating consists of cellulose acetate and a surfactant, which has an active ingredient selected from the group consisting of organic ethers. The surfactant is commercially available as IGEPAL CO-530 from Rhone-Poulenc of Cranbury, N.J. Although Celgard® 3407 has furnished design improvements to overcome the limited cycle life performance of the nickel-zinc batteries, Celgard® 3407 has an electrical resistance of less than 20 milliohms-inch2, and it may take up to 100 seconds to wet in aqueous electrolytes after 12 months of storage.
Accordingly, there is a need for an improved separator for a battery having a zinc electrode, which has a greater shelf life, i.e. a greater re-wet capability, and a lower electrical resistance.